The present invention relates to a signal amplifier circuit for electrical and electronic equipments used in automobiles, motorcycles and industries. Specifically, the present invention relates to a signal amplifier circuit for semiconductor physical quantity sensors used in automobiles and motorcycles.
In these days, the mechanical controls performed by mechanical parts are quickly being replaced by electrical and electronic controls performed by electrical and electronic parts in the fields of automobiles and motorcycles. Moreover, the conventional electronic parts are being replaced by more precise and multifunctional products for facilitating higher grade control.
For example, it is required for the pressure sensors, used for measuring the pressure inside the intake manifold or for measuring the brake oil pressure, to perform not only the pressure measuring function but also the function of detecting the troubles relating to the pressure measuring function (that is the self-diagnostic function, hereinafter referred to simply as the “diagnostic function”).
FIG. 10 is a block diagram of a conventional pressure sensor illustrating that the pressure sensor includes a pressure detecting section and signal amplifier circuit 10c. The pressure detecting section converts the pressure to an electrical signal by piezoelectric effects. The electrical signal is processed in signal amplifier circuit 10c and the processed signal is outputted from output terminal 02 of signal amplifier circuit 10c (that is also the output terminal of the pressure sensor) to an engine control unit (hereinafter referred to as an “ECU”).
The diagnostic function exhibited by the pressure sensor is the function of detecting the breakage of wirings (such as a wire bonding, a lead frame, and a harness) connecting the pressure sensor and the ECU to each other and transmitting the detected wiring breakage to the ECU. The diagnostic function realizes a fail safe function that detects a trouble, if present, and prevents larger troubles from occurring.
FIG. 11 is a diagram (graph) illustrating the relationship between the output characteristics of the pressure sensor and the diagnostic function. In FIG. 11, the X-axis represents the pressure (in kPa) that the pressure sensor measures and the Y-axis the output voltage (in V) outputted from the pressure sensor. The following brief descriptions will be followed later by more detailed descriptions.
The conventional pressure sensor does not exhibit any diagnostic function and outputs a certain voltage corresponding to the measured pressure. More specifically, the conventional pressure sensor exhibits only the function of outputting a certain voltage within the range between Vb and Vc (the steady output range), as shown in FIG. 11.
In contrast, the pressure sensor that includes the diagnostic function is designed not only to output a voltage within the steady output range but also to output a voltage lower than Va or higher than Vd (a voltage in any of the diagnostic ranges) in the event of a wire breakage. Because the ECU receives a voltage in any of the diagnostic ranges, it is detected that the pressure sensor is in trouble.
For realizing the diagnostic function in a pressure sensor, the following two technical means are necessary.
(1) A means for outputting a voltage in the diagnostic ranges when wiring breakage such as wire breakage and harness breakage is caused.
(2) A means for not outputting any voltage in the diagnostic ranges when the pressure sensor is in the normal state.
Here, the description of the means (1) will be omitted, since the means (1) is disclosed in the Patent Document 1 disclosed herein.
For the means (2), the conventional method employs most generally the saturation voltages of a signal processing circuit (operational amplifier 41 for signal outputting in FIG. 10).
FIG. 12 is a block diagram of a signal amplifier circuit used as a general signal processing circuit for a pressure sensor and illustrates wherein signal amplifier circuit 10c is essentially negative feedback amplifier circuit 40. Negative feedback amplifier circuit 40 includes a differential amplifier circuit 40a and resistor 46a. Differential amplifier circuit 40a includes operational amplifier 41 and four resistors 42, 43, 44, and 45. Signal amplifier circuit 10c includes input terminals including Vin+terminal 011 for positive input and Vin−terminal 012 for negative input. Signal amplifier circuit 10c also includes Vout terminal 02 working as an output terminal and third reference voltage supply 70 feeding an offset voltage. Resistors 44 and 45 are connected in series between Vin+terminal 011 and third reference voltage supply 70. The connection point of resistors 44 and 45 is connected to the non-inverting input terminal (+terminal) of operational amplifier 41.
Resistors 42, 43, and 46a are connected in series between Vin−terminal 012 and output terminal 41b of operational amplifier 41. The connection point of resistors 42 and 43 is connected to the inverting input terminal (−terminal) of operational amplifier 41. The connection point of resistors 43 and 46a is connected to Vout terminal 02.
The output voltage Vout of signal amplifier circuit 10c is obtainable approximately from the following formula, in which the voltage of Vout terminal 02 is represented by Vout, the voltage of Vin+terminal 011 by Vin+, the voltage of Vin−terminal 012 by Vin−, the resistance value of resistor 43 by R43, the resistance value of resistor 42 by R42, and the voltage of third reference voltage supply 70 by Vref 3.Vout=(Vin+−Vin−)×(R43÷R42)+Vref 3
In signal amplifier circuit 10c, the upper limit saturation voltage and the lower limit saturation voltage of the output thereof are determined by the upper and lower limit saturation voltages of operational amplifier 41 and the voltage drop caused across resistor 46a. In greater detail, the upper and lower limit saturation voltages of operational amplifier 41 and the voltage drop caused across resistor 46a depend on the following factors.
Upper and lower limit saturation voltages of operational amplifier 41
(1) The saturation voltages of the transistors used in the output stage of operational amplifier 41
(2) The impedance components of the transistors used in the output stage of operational amplifier 41 Voltage drop across resistor 46a 
(3) The resistance value of resistor 46a 
(4) The current flowing through resistor 46a (≈the load current flowing into and out of Vout terminal 02)
FIG. 13 is a diagram describing the lower limit saturation voltage and FIG. 14 is a diagram describing the upper limit saturation voltage.
Referring now to FIG. 13, the lower limit saturation voltage of the output is the sum of the voltage V6 of transistor Tr2 (MOSFET) in the output stage of operational amplifier 41 and the voltage drop V7 across resistor 46a. The voltage V6 of transistor Tr2 includes the ground potential, i.e. 0 V, which is the source potential of transistor Tr2, the ON-voltage of transistor Tr2 caused by the sink current I03 of operational amplifier 41, and the voltage caused by the impedance of transistor Tr2. The lower limit saturation voltage of the output is around 0.2 V.
Referring now to FIG. 14, the upper limit saturation voltage of the output is obtained by subtracting the voltage V5, consisted of the ON-voltage of transistor Tr1 (MOSFET) in the output stage of operational amplifier 41 caused by the source current I13 of operational amplifier 41 and the voltage caused by the impedance of transistor Tr1, and the voltage drop V8 across resistor 46a from the drain voltage of transistor Tr1, i.e. the power supply voltage VDD, which is around 5 V. The upper limit saturation voltage of the output is around 4.8 V.
As described above, the factors that determine the saturation voltages of the output from conventional signal amplifier circuit 10c depend on the characteristics of transistors Tr1 and Tr2 constituting operational amplifier 41 and the resistance value of resistor 46a. For example, as the resistance value of resistor 46a and the voltages V5 and V6 of transistors Tr1 and Tr2 lower, the lower limit saturation voltage lowers and the upper limit saturation voltage rises.
Contrary to this, as the resistance value of resistor 46a and the voltages V5 and V6 of transistors Tr1 and Tr2 rise, the lower limit saturation voltage rises and the upper limit saturation voltage lowers.
Since the lower and upper limit saturation voltages depend on the characteristics of transistors Tr1 and Tr2 constituting operational amplifier 41 and the resistance value of resistor 46a, as described above, variations are liable to be caused in the lower and upper limit saturation voltages due to the variations caused by manufacturing transistors Tr1 and Tr2 and resistor 46a (hereinafter referred to simply as the “manufacturing variations”) and the temperature dependence of the transistors' characteristics and the resistor's resistance value (hereinafter referred to simply as the “temperature dependence”). It is difficult to confine the lower and upper limit saturation voltages within small variations. Although the current flowing through resistor 46a includes a current I7 flowing from the load side to resistor 46a and a current I8 flowing out from resistor 46a to the load side as described by the broken lines in FIGS. 13 and 14, these current components are omitted from the above description. In FIGS. 13 and 14, Tr1 may comprise a p-channel MOSFET device.
The influences of the manufacturing variations and the temperature dependence on the output characteristics of the pressure sensor will be described below with reference again to FIG. 11. In FIG. 11, the reference for the pressure sensor output is represented by the solid line b. In other words, the saturation voltage ranges (which are the ranges, in which the voltage is constant and the constant voltage is the saturation voltage) are between the steady output range and the diagnostic ranges. The lower limit saturation voltage is located in the range (Δ1) between Va and Vb and the upper limit saturation voltage in the range (Δ2) between Vc and Vd.
A liner output voltage is obtained corresponding to the pressure in the steady output range. When an extremely high pressure (or an extremely low pressure) outside the steady output range is caused while the pressure sensor is conducting the steady operations (with no wiring breakage) not by a trouble but by any other factor, it is desirable for the pressure sensor to continue operating. For facilitating this, it is desirable for the output from signal amplifier circuit 10c to saturate before teaching the diagnostic ranges. In other words, it is desirable for the output from signal amplifier circuit 10c to saturate in the range between Va and Vb and in the range between Vc and Vd.
However, when the output voltage range of signal amplifier circuit 10c is widened (that is the output voltage hardly saturates) due to the adverse effects of the manufacturing variations and the temperature dependence, the output voltage as represented by the broken line a is obtained. Because the output voltage represented by the broken line a saturates in the diagnostic ranges, the saturation voltage may occur in any of the diagnostic ranges even when the pressure sensor is conducting the steady operations. In this case, the ECU will misdiagnose that the pressure sensor is in trouble.
Contrary to this, when the output voltage range of signal amplifier circuit 10c is narrowed (that is the output voltage saturates easily), the output voltage as represented by the broken line c is obtained. The output voltage may saturate in the region, in which the output voltage should depend linearly on the pressure (the region near to Vb or Vc). Therefore, the pressure sensor may fail to exhibit the designed functions.
One way of obviating the problems described above is to set the diagnostic ranges and the steady output range considering the ranges, for which the saturation voltages vary due to the manufacturing variations and the temperature dependence. In other words, it is effective to secure the range between Va and Vb and the range between Vc and Vd as widely as necessary to absorb the saturation voltage variations caused in the output characteristics.
However, the saturation voltage variation range of conventional signal amplifier circuit 10c is wide. FIG. 15 is a graph exemplary describing the temperature dependence and the manufacturing variation range of the lower limit saturation voltage in conventional signal amplifier circuit 10c. In FIG. 15, the X-axis represents the temperature (° C.), the Y-axis the lower limit saturation voltage (V), MAX the maximum values of the lower limit saturation voltages caused by the manufacturing variations, TYP the designed values of the lower limit saturation voltages, and MIN the minimum values of the lower limit saturation voltages caused by the manufacturing variations.
As FIG. 15 indicates, variations of around 70 mV are caused by the temperature dependence, variations of around 70 mV are caused by the manufacturing variations, and, therefore, the total variations of around 140 mV are caused in the lower limit saturation voltage of signal amplifier circuit 10c. In FIG. 15, the measurements are conducted at the power supply voltage of 5 V. Therefore, the total variations of 140 mV are caused for the power supply voltage of 5 V. In other words, the lower limit saturation voltage of conventional signal amplifier circuit 10c has a variation range equivalent to 2.8% of the entire output voltage range that is not necessarily narrow enough. Although not described in detail, similar variations are caused in the upper limit saturation voltage.
Patent Document 1: Unexamined Japanese Patent Application Publication 2003-304633.
For narrowing the output voltage variation range, it is necessary to reduce the manufacturing variations caused in the characteristics (the ON-voltage, impedance and such characteristics) of transistors Tr1 and Tr2 in operational amplifier 41 and resistance value of resistor 46a. Although the manufacturing variations may be reduced by selecting the characteristics of the pressure sensor or by innovating on the manufacturing method, the manufacturing costs will be increased. Since the temperature dependence relates to the temperature dependence specific to the relevant material, it is difficult to reduce the temperature dependence.
In view of the foregoing, it would be desirable to provide a signal amplifier circuit, substantially unaffected adversely by the manufacturing variations and the temperature dependence of each constituent element (the transistor or the resistor) and exhibiting small variations in the saturation voltages as the characteristic output thereof.
Further objects and advantages of the invention will be apparent from the following description of the invention.